The Wonders of Scorpius

Last week we highlighted the occultation of the bright red star Antares, of Scorpius, the Scorpion. This week, with the bright Moon now vacating our early evening sky, we'll highlight the entire star pattern that we've come to know as the Scorpion.

Large, famous and rich, it is the most striking and most easily learned of all the deep southern constellations seen from the United States; the most beautiful of all the zodiacal constellations. Just as Orion is the most striking winter stellar pattern, such a distinction can be claimed for Scorpius for the summer season.

One star clearly outshines the others, with a fiery tinge seemingly emphasizing the scorpion's sinister appearance. That's our old friend, Antares, a supergiant star, 700 times the Sun's diameter, large enough to engulf even the orbit of Mars, if the solar system were centered on it.

Antares itself is relatively cool as stars go; only 6,000 to 7,000 degrees Fahrenheit compared with 11,000 degrees for the Sun. Its low temperature accounts for its ruddy color. It's at least 17,000 times as luminous as the Sun, yet less than one-millionth in overall density.

Antares is 604 light years away and is an irregular variable star, meaning that it expands and contracts in an unpredictable manner causing its brightness to fluctuate. It has been observed to shine as brightly as magnitude 0.88, while at other times, its brightness has been down to 1.16. It usually ranks as fifteenth on the list of the twenty brightest stars in the sky. It has a small, very hot companion, bluish-white in color, but yet has been described as "a little spark of glittering emerald" because of its proximity and contrast to ruddy Antares.

To the ancients, its distinctive red color suggested the planet Mars and the name Antares means "The Rival of Ares," Ares being the Greek name for the God of War. Indeed, the planet Mars, although popularly called, "The Red Planet," really appears more of an orange-yellow color, whereas Antares glows with a distinctive ruddy coloration. So even when Mars is near opposition (as it will be in November of this year) and appears to glow many times brighter, Antares still rivals it at least in color.

Unfortunately, Scorpius is a star pattern that appears to slink across the southern sky, rather low and close to the horizon. With this in mind, it is well worth seeking out a good clear horizon to appreciate this majestic figure. The whole figure of the Scorpion is a magnificent sight and is best appreciated now in a dark sky without any interference from bright moonlight. It really looks like a huge scorpion; the upper stars of this star pattern form its body while its tail slants toward the horizon, then curves to the left and upward, a fine stream of stars ending in the wide pair -- Shaula and Lesath -- in the Scorpion's stinger.

In July 2000, the familiar head of Scorpius sported a new look as its middle star, Dschubba (from the Arabic for "the Scorpion's Forehead"), suddenly and unexpectedly brightened from its usual magnitude 2.3 to 1.9. The change was enough to make Dschubba quite plainly the brightest star in the naked-eye row of Graffias, Dschubba, and Pi Scorpii near Antares. It faded slightly a few months later, but when it emerged into the dawn sky in late December 2000, sky watchers found it still brighter than normal. It apparently reached its peak in 2003, reaching magnitude +1.6. It has since diminished to magnitude +2.0, which is still a bit brighter than normal. Dschubba is believed to be a rapidly rotating star, occasionally flinging mass out into space from its equator.

Scorpions have two large claws in front, but this one seems to have had his claws clipped; they were cut off to form the constellation of Libra. The Romans decided that there should be 12 constellations in the zodiac, instead of 11, so they made the two claws of Scorpius into the arms of Libra, the Balance.

Lastly, it must be emphasized to those newcomers of astronomy that the currently accepted name of this constellation is Scorpius, not Scorpio. Principally astrologers (and some older astronomy books) use the latter for labeling the zodiacal sign of that name.

Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.

DEFINITIONS

Degrees measure apparent sizes of objects or distances in the sky, as seen from our vantage point. The Moon is one-half degree in width. The width of your fist held at arm's length is about 10 degrees.

1 AU, or astronomical unit, is the distance from the Sun to Earth, or about 93 million miles.

Magnitude is the standard by which astronomers measure the apparent brightness of objects that appear in the sky. The lower the number, the brighter the object. The brightest stars in the sky are categorized as zero or first magnitude. Negative magnitudes are reserved for the most brilliant objects: the brightest star is Sirius (-1.4); the full Moon is -12.7; the Sun is -26.7. The faintest stars visible under dark skies are around +6.